explicit raise from

v6d1tokens/app.py

@@ -62,11 +62,11 @@ class V6D1TokensAppFactory(AppFactory):
                 requester = await requester_for_request(request, requester_cert, token_id)
                 requesterpk: PublicKey = requester.to_curve25519_public_key()
                 token_encrypted = await self.tdb.get_encrypted(requesterpk, token_id)
-            except BadSignatureError:
-                raise web.HTTPUnauthorized
-            except (json.JSONDecodeError, AssertionError):
-                raise web.HTTPBadRequest
-            except KeyError:
-                raise web.HTTPNotFound
+            except BadSignatureError as e:
+                raise web.HTTPUnauthorized from e
+            except (json.JSONDecodeError, AssertionError) as e:
+                raise web.HTTPBadRequest from e
+            except KeyError as e:
+                raise web.HTTPNotFound from e
             else:
                 return web.Response(body=token_encrypted)